Force transducers for control actuators convert external forces applied to the actuator by a user into corresponding electrical signals that are used to control a device or position an object of a display. The external forces are converted into electrical signals that represent force components along orthogonal axis. For example, a joystick type display controller for a computer includes a force transducer that converts forces applied to the joystick into two signals, one signal representing the component of force along an X axis and a second signal representing the component of force along a Y axis that is orthogonal to the X axis. The relative magnitude of the X and Y signals represent that relative amount of display pointer movement along the X and Y axis desired by the user. A computer to which the display controller is attached receives the electrical signals and moves the display pointer accordingly. The force transducer of the display controller is provided with electrical power and generates the electrical signals representing the force components as the external force is applied.
The joystick type display controller described above can include a force transducer comprising a lever arm that is mechanically coupled to a support base by an articulated joint. While the force transducer provides the needed force component electrical signals, the mechanical linkage needed to permit movement of the joystick in the desired directions is complex. In particular, the mechanical linkage is relatively large, bulky, and expensive. Also, the mechanical linkage is subject to wear and reliability problems.
As an alternative to the joystick force transducer with a mechanical linkage, force transducers have been developed with thin film resistive strain gauges. A thin film resistive strain gauge uses a conductive, thin film resistive strain gauge material that is deposited onto a thin flexible substrate using photo lithographic fabrication techniques. The strain gauge material undergoes a change in electrical resistivity when the underlying substrate is subjected to strain. If an electrical current is passed through the strain gauge material, the change in resistivity can be detected by an output voltage change and the relative amount of strain can be measured. Typically the substrate undergoes strain when forces are applied to the lever arm.
For example, the IBM Corporation ThinkPad.TM. 750 laptop computer is provided with a display controller force transducer comprising a pointer actuator that extends upwardly from a location between keys of the computer keyboard between the left and right hands of a computer user when such hands are resting at the keyboard home position. As forces are applied to the lever arm, the strain is detected and is used to control a display point on a monitor or display unit. This permits a user to control the associated display pointer without removing his or her hands from the keyboard. The user's hands otherwise would need to move from the keyboard to manipulate, for example, a display mouse controller or joystick controller.
The IBM Corporation Track Point.TM. system incorporated into, for example, the IBM Corporation ThinkPad.TM. computer consists of a set of strain gauges in a balanced bridge. By their nature the strain gages produce very small signals which is a function of the physical properties of the selected materials and the geometry utilized. Those signals must be amplified on the order of 300 times (300.times.) to be satisfactorily used in the TrackPoint system. With such large amplification, the direct current (DC) offset from the bridge, as well as any amplifier drift must be corrected in order to keep the input signal "centered" in the linear range of the amplifier. To accomplish this, the systems frequently use a relatively expensive electronic analog potentiometer that is adjusted periodically by the track point controller.